Modpack version
1.66c

Can the issue be reproduced?
Yes

Logs
N/A

Issue
Supremium Pickaxe does not have correct mining level

Has you pickaxe been modified with sharpening kits by any chance? If it has a sharpening kit with a lower tool material the item adopts that material for the mining level. Just checking.

It's a mystical agriculture Supremium pick, not tinker's

You're meant to use Tinkers exclusively, that's why the harvest level of "ordinary" tools is lower.